The giant garden slug Limax maximus (Limacidae, Pulmonata) is considered one of the most widely spread terrestrial molluscs in the world and is a formidable pest of agricultural and horticultural crops. This slug was recently introduced to Japan, where its population is now rapidly increasing and spreading. A naturalised population of L. maximus was first discovered in Hokkaido, Japan, in 2012, and the species has become common in this area. In the present study, we investigated observations of L. maximus reported by ordinary citizens acting as “citizen scientists” to assess the recent expansion of this invasive slug. As a result of 38 observations provided by local citizens, 16 naturalised populations of L. maximus were detected in Hokkaido. Moreover, a terrestrial macrophagous leech, Orobdella kawakatsuorum (Arhynchobdellida, Orobdellidae), is reported as a potential native predator of L. maximus for the first time.